To characterize the changes in pancreatic function during postnatal development, isolated pancreatic acini were prepared from rats aged 8–9, 12–14 and 20 days and from adult rats. Isolated acini maintained a normal microscopic appearance and viability as judged by exclusion of trypan blue and linear incorporation of tritiated leucine into total protein. The rate of incorporation in 8-day-old acini was 20% of that observed in adult rats. Significant dose-dependent increases in amylase release in response to carbachol were observed in all age groups; stimulated amylase secretion was significantly less in the 8- to 9- and 12- to 14-day-old animals than in the 20-day-old and adult rats. These data indicate that viable isolated pancreatic acini can be prepared from suckling rats and that these acini exhibit an altered in vitro responsiveness to carbachol. This preparation should therefore be a useful model for in vitro studies of the development of pancreatic function.